negative electrical ground

I was doing some basic maintenance on my motorbike this weekend which included checking the fluid level in the battery. On my bike, to check the fluid level, you have to remove the battery. So, I disconnected the cables from the battery. You always disconnect the negative terminal on the battery first so you don’t get sparks all over.

This brought up the age old question (for me, at least) WHY is there a negative ground in cars and bikes?

I was taught, way back in middle school that electricity flows from negative to positive in a DC circuit. I’ve heard there are a few heretical scientists out there that insist it’s the other way around, but everything I’ve read say electricity goes from negative to positive.

So, why’s it set up to, apparently, flow from positive to negative in a car or motorbike? All the elecltrical diagrams seem to show the electrical flow as going from positive to negative. Can any of the electrical engineers out there explain it?

The positive to negative flow concept predates the electron theory of current flow. In EE school you learn that it really doesn’t matter which you use, but the positive to negative precedent was set.
In many cases the current flow is due to positive ion flow rather than electrons, so to be perfectly accurate you would have to switch your equations around a lot anyway and no one wants to do that.

Regarding your question about why a negative ground is used on motorcycles and cars. Early on there was a mix of negative and positive grounds on vehicles. In the late 40’s & early 50’s the negative ground won out. I am not really sure why. Maybe because equipment with electron tubes used negative ground to the chassis.

In the 60’s/70’s, the Chief Engineer for I believe, White Motor Company - a truck manufacturer - carried on quite a campaign (unsuccessfully) for positive ground systems. As I recall, his argument was that due to ion flow, with negative grounds, the vehicle metal was plated to the battery thus eating away parts of the vehicle. He contended that it was better to plate the battery terminal to the vehicle metal than vice-versa since it was cheaper to destroy the battery over time than the metal of the vehicle.

And they still can’t decide what side to put the gas tank on? Bah.

Joe Lucas, Prince of Darkness, was famous for using a positive ground on many British cars.

Here’s some tips on how to switch over a Land Rover to a proper ground.

Though dead nearly a hundred years now, and largely not responsible for the agonizingly bad components that have been the curse of a million British sports car fans, Lucas is still the brunt of countless jokes. Here is one example from Electrical Theory by Joseph Lucas:

*Positive ground depends upon proper circuit functioning, the transmission of negative ions by retention of the visible spectral manifestation known as “smoke”. Smoke is the thing that makes electrical circuits work; we know this to be true because every time one lets the smoke out of the electrical system, it stops working. This can be verified repeatedly through empirical testing.

When, for example, the smoke escapes from an electrical component (i.e., say, a Lucas voltage regulator), it will be observed that the component stops working. The function of the wire harness is to carry the smoke from one device to another; when the wire harness “springs a leak”, and lets all the smoke out of the system, nothing works afterwards.*

The way I heard it, Positive Gground was a transient fashion that was supposed to cause wearing away of the central electrode of spark plugs rather than the side arm bit. Thus as the electrode wore away, the side arm could easily be bent down to keep the gap correct, whereas wear on the side arm produced a funny shaped cavity whose distance from the centre electrode became rather indeterminant and hard to measure.

These days the necessity of easily accommodating cheap electronic equipment (stereos etc) with their predominantly NPN (negative ground) transistors, easily over-rides any such “maybe” type theories.

Hardly anybody would want a postive ground vehicle these days because of the extra expense of adapting stereos etc to work in it.

Also these days, hardly any commercial maintainers re-gap spark plugs, they just sell you new ones; so that’s that theory out of the way.

**Diver wrote:

The positive to negative flow concept predates the electron theory of current flow. In EE school you learn that it really doesn’t matter which you use, but the positive to negative precedent was set.

In many cases the current flow is due to positive ion flow rather than electrons, so to be perfectly accurate you would have to switch your equations around a lot anyway and no one wants to do that.**

(scratches head) So… electrical current is really positive ions instead of just electrons? Positive ions flowing from the POSITIVE terminal and electrons flowing from the NEGATIVE terminal. And it doesn’t matter which way they flow, as long as there’s a complete circuit?

He didn’t say current flow is ALWAYS ion flow, just “in many cases”. Electrical current through a wire is electron flow. Electrical current through an electrolyte (such as inside a battery) is ion flow. Don’t ask about electron tunneling, or hole-conduction inside semiconductors :slight_smile:

Anway, the sign of the charge of the carrier (negative for electrons, positive for some ions) also affects the “direction” of current flow. You shouldn’t think of the “direction” of current flow as physically which way little pieces of charge are moving (it isn’t very useful to know), but rather which way signed charge is accumulating (very useful to know). A movement of electrons left-to-right in a conductor produces a current in the opposite “direction” as the same movement of positive ions, since electrons are negative. The EE convention is that current flow from the plus to the minus terminal of a battery is defined to be positive.

Here’s a quote from my old 200-level EE textbook that states the above better than I did:

It is convenient to think of current as the motion of positive charge even though it is known that current flow in metallic conductors results from electron motion. In ionized gases, in electrolytic solutions, and in some semicondutor materials, positively charged elements in motion constitute part or all of the current. Thus, any definition of current can agree only with the physical nature of conduction part of the time. The definition and symbolism we have adopted are standard.

It is essential that we realize that the current arrow doesn not indicate the “actual” direction of current flow, but is simply part of a convention that allows us to talk about “the current in the wire” in an unambiguous manner.



I’ve always wondered this myself. I believe (at least theoretically) it doesn’t matter whether you design an automobile’s electrical system to have a positive chassis or a negative chassis.

Of course, I think we can all agree that having cars with one system or the other would have disadvantages, since generic electrical components (such as stereos) would have to come in both “flavors.” Therefore, it is my WAG that everyone simply starting leaning toward one convention, i.e. chassis tied to negative.

Circuit in general are designed with the most negative point tied to ground. I’m simplifying here since many circuits have more than one power supply, and tie the center point (or some other point) to ground, and have power supply voltages above and below the ground. But, in general, single supply circuits such as a battery-powered circuit usually tie the negative point to ground. This convention makes the current flow out of the battery positive instead of negative (note that this isn’t the same “direction” as the actual, physical flow of electrons- see my previous post), which is convienent for EEs. There are some more concrete reasons, which boil down to differences in semiconductor transport of n-carriers and p-carriers (electrons and “holes”), that end up creating a preference for NPN transistors, etc., than end up creating a preference for negative-ground circuits.

None of this is directly related to automobile grounding, though, but if EEs are used to negative-ground, I can see where that would influence the more arbitrary choice in cars.